The overall goal of this proposal is to explore the involvement of non-classical MHC class Ib (class Ib) molecules in thymocyte differentiation. It has recently become more evident that in the absence of MHC class la (class la) expression, T cells can be selected by class Ib in the thymus. Not much is known about the physiological relevance of this alternative thymic selection process. The hypothesis I aim to test is that in the absence (or suboptimal level) of class la expression early in development, thymic differentiation and T cell education critically depends on class Ib. Both the thymic dependence of T cells and the factors that play key roles in the regulation of T cell development are broadly conserved among jawed vertebrates. Therefore, I propose to use the frog Xenopus and its immunocompetent but naturally class la-deficient larval stage, as a model because it provides the advantage of having the early developing thymus of larvae externally visible, easily accessible and amenable to experimental manipulation. Importantly, our hypothesis involving class Ib can be tested in the natural absence of class la and the development of T cells can be studied in a biological context. This proposal will provide fundamental, and not phylogenetically restricted, insight on the relative contributions of class la versus class Ib-mediated thymocyte education pathways during ontogeny.